An electrolytic process for the removal of sulfur dioxide from waste gases and recovery as sulfuric acid is described in U.S. Pat. No. 4,830,718. This patent discloses a process for removing sulfur dioxide from effluent vent or flue gas by subjecting the gas cyclically to scrubbing in an acid stream and to electrolysis. The process comprises the steps of scrubbing the gas in a confined scrubbing zone with an aqueous sulfuric acid stream to remove sulfur dioxide from the gas and converting the thus removed sulfur dioxide to sulfurous acid, subjecting the sulfuric acid stream containing the thus produced sulfurous acid to electrolysis in an electrolytic cell to oxidize the sulfurous acid to sulfuric acid, recycling the sulfuric acid stream resulting from the electrolysis step to the scrubbing zone, and maintaining the recycled sulfuric acid within a predetermined range of concentrations by means of make-up water or acid.
The scrubbing zone conveniently is a scrubbing column of conventional design or modified design for passage of a stream of effluent gas therethrough. Preferably, the scrubbing column contains packing material that provides gas-liquid contact surface for the gas stream and the aqueous acid stream. The design of the packing material is critical in order to minimize channeling and thereby achieve greater scrubbing efficiency.
In a favored embodiment of the process, the packing material is electrically conductive, and it serves both as the gas-liquid contact surface for scrubbing and as the electrochemically active surface of the anode of the electrolytic cell. With this configuration, the cathode is located in the electrolytic cell compartment external to the scrubbing column. Electrical contact between the anode and cathode is maintained by the aqueous acid stream flowing through the scrubbing column and the connecting piping leading from the column to the electrolytic cell.
The above configuration has obvious advantages. Hydrogen gas produced at the cathode is kept isolated from the flue gases by means of the aqueous acid seal in the connecting piping. Thus, hydrogen gas cannot intermix with the flue gases forming potentially explosive mixtures. Furthermore, the hydrogen gas formed in the process can be recovered as a useful byproduct for such applications as the production of ammonia. Alternatively, the hydrogen can be burned as a fuel in such general uses as steam generation, or it can be used specifically to reheat the flue gases thereby providing buoyancy to the gases for better dispersibility in the atmosphere.
In practice, however, the methods described for removing sulfur dioxide have serious disadvantages. When both electrodes are installed in the electrolytic cell compartment, large quantities of acid must be recycled between the electrolytic cell and the scrubbing column. Large liquid flow rates are necessitated because of the limited solubility of sulfur dioxide in aqueous acid solutions. This drawback is overcome when the column packing material serves not only as the gas-liquid interface but also as the anode of the electrolytic cell. The absorption equilibrium is shifted to the right by the electrolysis of sulfurous acid to the sulfuric acid.
New problems arise, however, when the column packing material is used for the anode. With this modification, the electrical resistance of the aqueous acid solution between the electrodes is increased significantly. In addition, there results troublesome variations in the voltage potential at different column heights. In large installations, no electrochemical reaction may take place at certain points in the scrubbing tower for lack of sufficient potential. If the voltage is increased, unwanted side reactions can occur at other locations thereby consuming excessive amounts of power. Another concern is the danger of grounding out the electrical current passing through the interconnecting piping. Providing electrical insulation for the lines circulating the aqueous acid solution can prove to be difficult.
It is, therefore, an object of the present invention to provide a process that overcomes the disadvantages of the conventional methods for removing sulfur dioxide from waste gases.
It is also an object to provide a process that is completely safe to operate and which will be acceptable under the most stringent regulations.
A further object is to provide a process requiring the minimum investment and offering the lowest operating costs.
These and other objects, features and advantages of the invention will be apparent from the accompanying drawings, FIGS. 1 and 2, and the following description.